Cryogenic devices have been provided heretofore for a variety of purposes and the particular type of cryogenic devices with which the present invention is concerned, is a device which can be used for conducting low temperature experiments, for subjecting samples, specimens and test objects to low temperatures and for treating materials with low temperatures selected between say 0.degree. C. and the boiling point of a liquid cryogen, i.e. a liquefied gas such as nitrogen or air. It is desirable with devices of the latter type to enable them to operate at any preselected temperature within this range and to hold the temperature stable for long periods of time.
Such devices are required for medical, scientific or industrial research and have generally comprised a vessel containing liquefied air or liquefied nitrogen into which the object to be treated is immersed directly or indirectly, i.e. in another sample-holding vessel so that the object is eventually able to be brought to a temperature approximating the boiling point or liquefaction point of the cryogen.
Obviously, if the temperature is determined only by the boiling point of the liquid cryogen, the device is suitable only to maintain this temperature, the liquid cryogen being replaced as it evaporates.
Thus the temperature which can be achieved with such a device is always the lowest temperature which results from the evaporation of the particular liquefied gas.
In order to adjust the temperature of the sample it has been proposed to surround the sample tube with an electric resistance heater which can supply to the sample the calories required to maintain it at a temperature above the boiling point of the free liquid in the vessel. The result is a thermal equilibrium between the liquefied gas and the sample controlled by the resistance heater.
Devices of this type have been found to be somewhat imprecise as a result of the poor distribution of heat to the sample from the resistance heater. Thus lower portions of the sample, bathed in the liquefied gas, are generally at a substantially lower temperature than upper portions of the sample surrounded by the electrical resistance heater. The thermal exchange between the two parts of the sample is certainly not instantaneous and frequently is relatively slow.
To obtain a better distribution of temperature, it has also been proposed to apply the resistance heater not only along the portion of the sample tube above the free surface of the liquid, but over the entire area of the sample tube. While this successfully improved the distribution within the sample, it, too, has a significant disadvantage. It is found, for example, that the thermal expansion and contraction to which the heater is subject because of the large temperature differentials to which it is exposed each time a sample is to be cooled, damage the heater. In fact, the mere energization and deenergization of the heater will bring about similar expansion and contraction to the detriment of this fragile unit.
In both of the cases described, the possibility of controlling the temperature is found to be limited also by the thermal inertia of the mass of liquid in which the sample is immersed. The liquefied gas has frequently far more mass than the sample and thus attempts to control finely the sample temperature are impeded by the thermal inertia of the liquid mass.
In another known system, the electric resistance heater for controlling the temperature surrounds the sample tube and the sample tube is not permitted to contact the liquefied gas directly. The resistance heater lines the inner wall of a receptacle in the center of which is placed the sample holder. This receptacle is, in turn, immersed in a much larger vessel containing a fixed volume of the liquefied gas, the latter being replenished as evaporation occurs.
Here again control of the temperature is obtained by supplying calories to a greater or lesser extent to balance the heat abstracted by the liquefied gas. While mechanically this system is more desirable than the earlier systems, the thermal energy which must be introduced to maintain a given temperature or for a given sample, is significantly higher.
Furthermore, the receptacle containing the resistance heater and the sample is always at atmospheric pressure and ambient air, carrying moisture, can diffuse into this receptacle or can be drawn into the latter by convection currents, thereby depositing ice on the sample tube or elsewhere in this receptacle.
Thus prior art devices for the purposes described have not been found to be fully satisfactory.